Method for the wet-chemical transformation of biomass by hydrothermal carbonization

ABSTRACT

A method for converting biomass into higher-energy-density solids, in particular carbon, humus or peat, is described. In the method, organic substances from the biomass are suspended in water to form a suspension and at least a part of the suspension to be converted is heated to a reaction temperature and is converted into higher-energy-density solids by hydrothermal carbonization at elevated pressure. The conversion is carried out in a reaction volume which is located underneath the Earth&#39;s surface. Uniformity of the product quality and an increase in the economic efficiency of the process are achieved by the method.

TECHNICAL FIELD OF APPLICATION

The present invention relates to a method for converting biomass into higher-energy-density solids, in particular carbon, humus or peat, wherein organic substances from the biomass are suspended in water to form a suspension and wherein at least a part of the suspension to be converted is heated to a reaction temperature and is converted into higher-energy-density solids by hydrothermal carbonisation at elevated pressure. The organic substances can be plant parts, other biomass or organic waste.

PRIOR ART

The conversion of biomass into products having a higher mass-specific energy content compared with the biomass used such as, for example, oil, gas or coal, is becoming increasingly important.

Known inter alia are methods for obtaining gas and/or oil and carbon at high temperatures, for example, by pyrolysis, gasification or sulphurisation. In this connection catalysts are frequently used to accelerate the reaction and positively influence the product composition.

Recently wet-chemical methods such as hydrothermal carbonisation have also been discussed for obtaining products having a higher mass-specific energy content compared with the biomass used. In this case, plant parts or other organic substances are comminuted, suspended in water and usually reacted with at least one conversion-promoting substance, for example, with acid and/or an additional organic and/or inorganic catalyst. The suspension is poured into a reactor and the reactor is closed. The suspension is then heated to temperatures between 170° and 250°. Since the reactor is closed, as a result of the water vapour partial pressure, the pressure increases with increasing temperature. Depending on the temperature, the pressure increases to values of 10*10⁵ to 20*10⁵ Pa (10 to 20 bar) or higher. In the course of the hydrothermal carbonisation reaction, hydrogen and oxygen are separated in the form of water from the carbohydrates contained in the biomass, whereby energy is released. The longer the reaction lasts, the more water is separated and the energy density of the products increases further. Solids such as, inter alia, peat, humus, lignite, low-grade anthracite or other substances having a significantly higher mass-specific energy content compared to the biomass used are produced.

The reaction proceeds faster or slower depending on the concentration and structure of the contents, primarily the carbohydrates (e.g. sugar, starch, cellulose, hemicellulose or others) in various raw materials, various plants and plant parts, residue from food production, sewage sludge or other biogenic materials and waste. Depending on the properties and the concentration of the biogenic raw materials used, more or less heat is released per unit time, the temperature and the pressure in the reactor increase more rapidly or more slowly and different absolute values are achieved for pressure and temperature. In the course of the reaction when increasingly less biogenic material is available for the reaction, the reaction is slowed considerably. The temperature drops again until the reaction is terminated after a certain time because the temperature is too low. This possibly leads to incomplete conversion of the material used, for which a reaction time of several hours up to several days or longer may be necessary. However, external after-heating of the reactor to lengthen reaction times requires an additional energy input which can make the carbonisation uneconomical.

The coupling of temperature and pressure in this reaction and the different reaction rates of different raw materials used have the result that the temperature and pressure profiles during the reaction are very different depending on the composition and concentration of the biomass in the input stream of the hydrothermal carbonisation. The products obtained can thus differ very substantially in their composition. This can have the result that the products have no constant quality and do not give high yields which can make the hydrothermal carbonisation uneconomical.

The object of the present invention is to provide a method for converting biomass into higher-energy-density solids by hydrothermal carbonisation whereby a more uniform product quality is achieved and the economic efficiency of the process is increased.

DESCRIPTION OF THE INVENTION

The object is achieved by a method according to claim 1. Advantageous embodiments of the method are the subject matter of the dependent claims or can be deduced from the following description or the exemplary embodiment.

In the method for converting biomass into higher-energy-density solids, in particular carbon, humus or peat, organic substances from the biomass are suspended in water to form a suspension. In a preferred embodiment a conversion-promoting substance is present in the water or is added to the water or the suspension. The conversion-promoting substance can, for example, be an acid and/or an organic or inorganic substance which accelerates the reaction. The biomass can comprise, for example, organic waste, plant parts, wood, algae or other carbon-containing organic products. At least a part of the suspension to be converted is heated to a reaction temperature and is converted into higher-energy-density solids by means of hydrothermal carbonisation at elevated pressure. The method is characterised in that the conversion is carried out in a reaction volume that is located underneath the Earth's surface.

The reaction volume for buffering released reaction heat in a surrounding area corresponding to at least four times the mean diameter of the reaction volume is preferably surrounded by a mass of compact liquid and/or solid material which is greater than eight times the mass contained in the reaction volume. Good homogenisation of the product properties is already observed above this mass.

In the proposed method the reaction therefore takes place underneath the Earth's surface. The Earth's surface is understood in this context as the boundary layer between the solid Earth's crust or the oceans on the one side and the atmosphere on the other side. By carrying out the process underneath the Earth's surface with a sufficient quantity of compact, liquid and/or solid material around the reaction volume, even with fluctuating biomass concentration and composition it is possible to produce energy- and carbon-rich products which have a significantly more uniform composition than products produced in the known manner in a reactor above the Earth's surface.

By moving the reaction chamber below the Earth's surface with the surrounding material such as, for example, rock, sand, water or soil, this material can absorb a large part of the energy released in the form of heat at the initial stage of the reaction. As a result of the heat exchange taking place with the surrounding mass, the temperature in the reaction volume or reaction mixture increases more slowly and not so far as in a reactor above the Earth's surface, and the pressure likewise does not fluctuate so strongly. In consequence at the beginning, the reaction does not proceed so rapidly and is more uniform. However, as the concentration of convertible biomass contents decreases with time, the temperature in the reaction does not fall so rapidly as in the hitherto known process. Rather, the surrounding material then slowly delivers the stored heat back to the reaction volume. The reaction volume thus remains warm for much longer and the reaction can be continued without additional heating of the suspension for many hours or even days until different raw materials have been converted to comparable products having a higher energy density. The reaction volume is preferably in direct contact with the surrounding compact material, at least in some places.

Another advantage of the proposed method is that by returning heat from the surrounding material into the reaction volume even after removal of the products, new biomass can be supplied again and brought to reaction without external heating or at least without strong additional heating. In many cases, this allows several batches to be carbonised successively without any external supply of heat. In principle, the method therefore allows both a continuous supply of biomass and also batch operation. As a result, the throughput in the method according to the invention can be varied very substantially as a result of the thermal buffering of the surrounding soil or water without any losses of uniformity in the product quality.

At the same time, the method must be carried out in a region underneath the Earth's surface in which a sufficient mass of surrounding material is available for thermal buffering. The material should preferably have such a compact structure that in the surrounding area of four times the mean diameter of the reaction volume, it should have a total mass which corresponds to eight times the mass contained in the reaction volume. Relative to a cylindrical reaction volume of diameter D and height H, this means that a cylindrical volume having the same height and four times the diameter minus the cylindrical reaction volume should contain at least eight times the mass contained by the reaction volume filled with the suspension in order to achieve particularly good buffering for the proposed method.

As a result of the pressure prevailing underneath the Earth's surface, the surrounding material such as, for example, soil, loam, sand or water is capable of at least partly compensating for and absorbing the pressure coming from the reaction. A reactor used for hydrothermal carbonisation underneath the Earth's surface can therefore be designed as considerably thinner-walled compared with that for use above the Earth's surface. This additionally saves costs. In a particularly simple reactor design, this reactor can, for example, consist of steel which is embedded in concrete or -reinforced concrete in a cavity under the Earth's surface. Very good heat transfer to the surrounding material takes place through the concrete cladding. The wall of this reactor can be very thin-walled.

Furthermore, the wall of the cavity can be used as the reactor wall. If necessary, this wall can be additionally lined with watertight materials. Such a lining can also be achieved by synthetic additives in the water. Automatic sealing by the reaction products of the process such as, for example, coal particles can possibly take place with respect to the surrounding rock.

The product composition can additionally be homogenised if the pressure is increased above the pressure corresponding to the reaction temperature. As a result of the additional application of pressure in the reaction volume, the pressure certainly increases during the entire reaction and then falls again but the percentage relative pressure fluctuations are smaller. It is particularly advantageous if the pressure in the reaction volume is kept constant or at least largely constant by means of technical measures. By means of these measures temperature and pressure are decoupled from one another. The operator of the hydrothermal carbonisation is therefore in a position to select the pressure according to the composition of the input so that the homogenisation of the product quality is improved. Application of an additional pressure can not only homogenise the composition of the end product. Rather, depending on the input material, the yield of solid products having high energy density can be increased by the elevated pressure so that the process can be operated even more economically. With the additional build-up of pressure, the operator has a valuable instrument at his disposal that can specifically vary and thereby optimise the product quality or the yield depending on the requirement and composition.

In addition to various other mechanical methods, for example, the pressure build-up can be achieved by moving the reaction volume sufficiently deep into the ground. The location of the reaction volume is selected to be sufficiently deep that a water column located above the reaction volume which is required to supply and remove the suspension, produces a hydrostatic pressure in the reaction volume which is higher than the equilibrium pressure which would be established at the reaction temperature in a gastight reactor filled with the suspension. During the build-up of such a hydrostatic pressure it is additionally very simple to maintain a constant pressure. In this case, it is merely necessary to ensure that liquid can enter or exit at the surface of the water column. This can be made possible, for example, by openings or by using non-sealing pumps such as, for example, rotary pumps. During a temperature rise in the reaction volume, liquid can then exit at the surface and the pressure in the reaction volume remains largely constant. The water column is thereby used as a pressure buffer. The reaction conditions are thereby homogenised and the solid material yield can be additionally increased.

Particular advantages are achieved if the reaction volume is configured to have a greater width than height. During the generation of hydrostatic pressure an approximately equal pressure is thus generated at all points in the reaction volume, thereby additionally promoting homogenisation of the reaction conditions. The reaction volume can be formed by insertion into horizontal shafts, for example coal shafts.

In a particularly advantageous embodiment for generating the hydrostatic pressure, a height difference of at least 100 m is selected between the upper filling level and the reaction volume. A pressure higher than 10*10⁵ Pa (10 bar) is thus formed in the reaction volume as a result of the water column located thereabove. Larger height differences of 200 m or more allow higher pressures to be established which can be very advantageous depending on the requirement.

In a very advantageous embodiment, the reactor is designed so that an inlet and outlet opening are located at the same height or at least at a similar height compared to the total reactor height so that the hydrostatic pressure difference between the openings does not exceed 10% of the pressure. The pumps used then do not need to overcome any high pressure differences and can thus be designed very simply and cost effectively.

In addition to using a rigid reactor, it is also possible to configure the outer wall of the reactor as flexible so that this nestles against the inner wall of the cavity or at least serves as a barrier towards the surrounding rock or water. Thin metal sheets or metal films can be used particularly advantageously here, these having a high temperature resistance compared to other materials.

An advantage of the pressure build-up by the hydrostatic pressure in the reactor is that the pressure increases uniformly with increasing depth. The reaction therefore does not begin abruptly and spontaneously but slowly and uniformly with increasing pressure and increasing temperature. By varying the delivery rate and the relative borehole diameter in the case of a borehole as the cavity, the dwell time can be specifically adjusted and thereby matched to the respective raw material. It is appropriate to attach cooling water connections at regular intervals over the height and volume of the reactor so that cold water can be introduced if required to slow the reaction. This can avoid overheating of the reaction and the heated water can thereby be used for energy. This process can also take place via heat exchangers to be installed in the reactor.

When adjusting long residence times in the reactor, it can occur that the flow rate needs to be throttled to such an extent that the particles in the suspension settle out more rapidly than the suspension flow. Different strategies are possible to avoid blockages in the reactor.

Thus, mixing elements, flow baffles, static mixers, agitators or other devices which influence the flow can be installed in the reactor to limit the sedimentation of solids. Gases such as, for example, compressed air can be particularly advantageously introduced into the reactor to effect thorough mixing. It is also possible to achieve gas formation by making the water in the suspension partially evaporate. The turbulence thereby produced leads to thorough mixing and avoids blockages.

Specific evaporation of part of the water can also be used to empty the reactor after the end of the reaction. For this, a pressure reduction in the reaction volume can be achieved by pumping away water located in the inlet or outlet and spontaneous evaporation is achieved in the reaction volume. As a result of this evaporation process which proceeds very similarly to delayed boiling, so much mass is conveyed from the reaction chamber in a very short time that as a result of the very high flow rates which thereby occur, sedimented or deposited solids and smuts are conveyed to the surface.

In order to increase the flow rate in the reactor, the flow cross-sections can be reduced to such an extent that a critical flow rate is exceeded. It is also possible to erect a cascade of agitator reactors in tunnels underground and surround these with soil, rock or water, through which flow takes place in series. In this form, they can be manufactured very cost-effectively and allow rapid flow.

Blockage of the reactor can also be avoided if the flow direction is reversed at regular intervals and thus a type of pulsation is achieved on which a constant flow rate is superposed. This pulsation leads to turbulence in the reactor and thereby very efficiently prevents deposits.

In order to largely avoid disturbing components in the reaction chamber, it is advantageous to remove disturbing components such as stones, metal, glass or similar inorganic materials from the suspension before the reaction. This can comprise gravitational separation such as a primary settling tank in sewage treatment works or a hydrocyclone or another method known from the prior art for separating solids from suspensions.

Furthermore, particles which tend to sediment can be specifically removed from the reactor. For this purpose apparatus can be incorporated in the reactor which discontinuously or continuously conveys sedimented solids from the reactor using systems according to the prior art (e.g. conveyor belts, scrapers, chains, screws, pumps). These solids can be fractionated outside the reactor so that coarse organic materials can be returned to the reaction chamber following appropriate comminution.

It can be desirable to remove from the reactor gases or vapour formed as a result of the pressure drop in the part of the reactor through which upward flow takes place. This can be achieved, for example, by perforating the reactor wall, for example in the part of the reactor through which upward flow takes place. These holes can be provided to the surroundings or to the in the part of the reactor through which downward flow takes place. Pressure compensation is thus ensured. Fissured or karst rock formations can also be used to remove the gas.

If a rapid escape of gases or liquid volumes due to temperature-density differences (geyser effect) is to be avoided, appropriate pressure valves or check valves should be incorporated which close when a defined pressure or a defined flow rate is exceeded, thereby briefly effecting a pressure rise and terminating the outgassing process. Convection flows can be specifically prevented by this means.

In a particularly simple design the reactor merely consists of a cavity present in deeper layers of rock where the supply of reaction mixture is conveyed through a supply line to a sufficient depth for the reaction. It is particularly advantageous, for example, to use old conveyor shafts from mining, disused tunnels or other underground structures. In this case, the existing lining of the shafts or tunnels can be used as the “reactor wall” and the entire volume of the shaft can be used as the reactor. In addition to lining the shaft with watertight materials, the system can be sealed by additives in waters or the system can seal itself with respect to the surrounding rock by reaction products such as coal particles.

When using bores in the ground, an inlet or outlet must be provided in the lower region of the reactor. By providing an inlet or outlet channel in the lower region of the shaft or the bore whose cross-sectional area and pump capacity can be variably adjusted, an upward flow is established over the remaining shaft cross section. Depending on the requirement, the area ratio of reactor space through which upward flow takes place and reactor space through which downward flow takes place can be 0.01% to 99.99%. Heat exchangers for cooling or heating, which can be arranged in the shaft, are used to control the temperature and the reaction and thereby ensure the product quality and the energy removal and consequently the energy utilisation outside the reactor.

It can be desirable to convey the product flow upwards from the depths at another location from that at which the raw material was introduced, similar to the use of geothermal energy. For example, horizontally running coal shafts which were previously used as underground mining areas and are now disused, can be used as reaction volumes. Thus, the raw material suspension can be introduced into the shaft via externally located regions and all input flows from the conveyor shaft can be conveyed centrally out from the depths or the other way round. Thus, existing shaft installations can be almost completely used as “reactors” for the production of biogenic fuels. Many tens of thousands of cubic metres of volume are available there as reactor so that very high throughputs can be achieved despite the long dwell time of the reaction. It is also possible to use other underground cavities with gas or water, caverns, caves, karst and porous rock formations or water-filled tunnels as reactors, The person skilled in the art from the field of geology will be able to identify suitable cavities which can be flooded with water and used for the method described.

In this case it is always particularly advantageous to use the Earth's heat at greater depths to promote the reaction. It can also be appropriate to use residual deposits of coal or oil. Here in parallel with the biomass reaction, it is also possible to extract the residual reserves from oil deposits which have already become uneconomical to extract as a by-product, so to speak. Thus, the volume of the deposit can be used as the reactor and remaining residual deposits of fossil raw materials can be appropriately utilised. As a result of the high reaction temperature, oil in the rocks is thin so that the residual deposits can be conveyed very efficiently from the depths.

It is advantageous to cycle the water used for the suspension completely or partially in the cycle to completely utilise the raw material. For this purpose it is necessary to separate the desired reaction products such as, for example coal particles, from the suspension and to convey the remaining substrates, unreacted raw materials and reaction products such as, for example, phenols or other secondary products together with new comminuted biogenic raw material back into the reaction chamber. It is known to the person skilled in the art that a concentration of minerals or non-convertible fractions is to be avoided. This can be achieved by a suitably dimensioned bleed flow.

Process-technology solution approaches such as secondary sedimentation basins, decanters, filter presses or briquetting installations can be used to separate coal particles.

Another possibility for implementing this invention can be seen in the submarine area. Here, for example, simple thin-walled conduits can be guided into the sea at great depths as reactors. However, it can also be advantageous to introduce the reaction mixtures directly into deep layers of lakes or deep sea layers and therefore use large areas of the sea such as, for example, coastal areas with great sea depths as reactors. In this case, it can be particularly advantageous to use regions in the ocean which are particularly hot due to volcanic activity such as, for example, some regions in the Pacific. The upward flow of hot submarine springs can then be used to convey the reaction products such as, for example, coal.

The method described brings additional advantages when used in combination with geothermal energy. Thus, energy is supplied to the reaction mixture in warmer regions of the Earth, the reaction mixture is additionally heated and the reaction thereby accelerated. The additionally released energy can then be used according to the prior art by removing the heat or by converting into electrical current or hydrogen.

The carbon-rich reaction products are present in many cases as finely dispersed nanospheres. This circumstance can be used very advantageously for conveying the solid energy carriers. Thus, after the energy carrier suspension emerges from the reactor, mechanical separation of the solids from the liquid can initially be carried out, for example, by centrifugal separation methods. The liquid fraction containing the amino acids and minerals from the organic raw material can be used as manure directly or after concentrating by partial separation of the water. The nanoparticle solids which predominantly consist of carbon are again mixed with water and adjusted to a dry substance content of 40 to 60 mass %. An energy density of up to 18 gigajoules per tonne can thus be established in the suspension, which corresponds to approximately half the energy density of crude oil. Transporting nanoparticle energy carriers over large distances in this form is completely economical by pipelines known from the prior art.

The viscosity plays a decisive role in avoiding sedimentation effects and for the separation of coal from the liquid phase. In order to avoid too-rapid sedimentation of the biogenic raw materials, the viscosity of the suspension in the reactor inlet should be at least 20 mPas (measured in a rotary viscosimeter at a shear rate of 10 m/s). In the reactor outlet, values of 5 mPas should not be exceeded for improved separation of particulate solids.

Various methods are known to the person skilled in the art for adjusting the viscosity in the inlet. Thus, an elevated viscosity can be set by specifically using different biomasses, biomass having defined carbohydrates (cellulose, starch, oligo- or monosaccharides), their degree of comminution, concentration and via the swelling time of the carbohydrates. In this case, the parameters described and the choice of raw material should be varied in such a manner that after the reaction, the liquid has a correspondingly low viscosity as a result of the degradation and conversion of the biomass.

In a further advantageous embodiment of the method according to the invention, the biomass or the reaction suspension is conveyed into the depths in vessels or drums such as containers, barrels, baskets, sacks, cylindrical or rectangular vessels made of different materials or in similar spatially defined volumes, into the interior of the reactor accommodated underground.

As a result of the high thermal capacity of the ground and the liquid in the reactor, the containers or the vessels in the reactor are sufficiently heated so that the reaction can take place inside the container without removing the biomass from the containers.

This measure can very efficiently avoid particles from the suspension sedimenting on the bottom of the reactor and no longer being able to be removed therefrom. In the embodiment of the closed or half open container, the particle size can be variably adjusted or fine comminution can be dispensed with before carrying out the reaction. Thus, larger particles such as, for example, pieces of wood can be conveyed into the reactor. Coal particles having an edge length of several centimetres can thus be obtained, making it easier to separate the water from the coal after completion of the reaction.

It is important for the embodiment using containers that provision is made for pressure equalisation with the surrounding water. It must be ensured by providing openings or valves in the containers that according to the depth, the pressure in the reactor can be transferred to the interior of the container.

It can also be advantageous to allow an exchange of liquid from the container with the surrounding liquid through suitable openings in the container. The heat transport into the interior of the container is thereby improved and the transfer of liquid reaction products and non-suspended extremely fine particles from the containers into the surrounding liquid and therefore also into other container is made possible. Thus, despite very efficient avoidance of sedimentation effects and blockages in the reactor, exchange of temperature, liquid and reaction products can take place between the containers, resulting in a faster reaction and homogenisation of the product composition.

The containers can be conveyed through the reaction space similar to the situation with tower heaters in the food industry which are used for heating tins in a displacement conveyance system, i.e. each container pushes the next container further in the tubular reaction space. It is also possible to use conveyor systems for containers according to the prior art such as, for example, chain conveyors, conveyor screws, cables and other devices for transporting vessels through conduits. The containers can also be transported through the shaft or reaction space in like manner to other transport systems in mining by cables or on rails in a type of “underground railway”.

It is also possible to produce spatially defined reaction volumes by separating individual reactor regions by using locks, metal sheets or other internal parts or by so-called scrapers which largely seal the reactor cross section and can be entrained with the flow as dividing walls between individual reactor sections. As a result of this multichamber design of the reaction space, different process conditions such as temperature, for example, can be established in each segment, making it easier to control the process.

BRIEF DESCRIPTION OF THE DRAWINGS

The proposed method is explained briefly hereinafter with reference to an exemplary embodiment in conjunction with the drawings. In the figures:

FIG. 1 is an example of the arrangement of the reaction volume underneath the Earth's surface; and

FIG. 2 is a schematic diagram of the process sequence in the proposed method.

WAYS FOR IMPLEMENTING THE INVENTION

FIG. 1 schematically shows an example of an embodiment of a reactor for carrying out the present method, which in this example is inserted in a shaft 1 underneath the Earth's surface. The shaft 1 lies at a depth of 200 m. The reactor 2 has an inlet 3 to the reaction volume which in this case occupies the entire volume of the horizontally arranged reactor. The suspended biomass is pumped via this inlet 3 into the reaction volume. The reaction products are pumped upwards again via the outlet 4. The wall of the reactor 2 can be relative thin since the hydrostatically generated pressure in this case is absorbed by the surrounding soil 5. The reactor 2 is surrounded in an area of soil 5 which corresponds to at least four times the diameter D of the reaction volume. No larger cavities can be present in this surrounding area so that the total mass in a volume occupied by the material in this surrounding area corresponds to at least eight times the mass of the reaction mixture in the reaction volume. The suspended biomass is initially brought to a temperature of about 80° C. in the reactor 2. As a result of the very violent exothermic reaction in the reaction volume at the beginning of the process, the suspension is heated above 200° C. As a result of the large mass of the surrounding material, the heat absorption and storage has the result that no rapid overheating takes place. In a subsequent course of the reaction when substantially less heat is produced, the reaction temperature is achieved by the heat released by the surrounding material, so that the reaction can be maintained for a fairly long time without any external supply of energy.

FIG. 2 schematically shows the process sequence again in a flow diagram. The biomass 6 supplied from a farm, which can be in the dry or wet state, is initially comminuted in a comminution and suspension step 7 and suspended in water. Acids, organic and inorganic catalysts can be used as additives. After heating the suspension thus obtained to about 80° C., this is conveyed by means of a suitable pump into the deep shaft reactor 8 as shown schematically, for example, in FIG. 1. The exothermic reaction takes place in the reaction volume of this reactor whereby in the first time interval of the process, a hot suspension at about 200° C. containing water and coal particles is removed from the reactor. The heat of this suspension is used in a conversion step 9 to produce electrical energy. In a separation step 10 the water and coal are separated so that finally pure coal 11 is available for energy production. The coal can be used, for example, as raw material for liquid hydrocarbon-rich fuels. In the separation step 10 a fraction comprising water with minerals and amino acids dissolved therein is obtained. The minerals and amino acids are separated in step 12 and transported back to the fields again as manure. The water is reused in the comminution and suspension step 7.

REFERENCE LIST

-   1 Shaft -   2 Reactor -   3 Inlet -   4 Outlet -   5 Surrounding soil -   6 Biomass -   7 Comminution and suspension step -   8 Reactor -   9 Conversion into electrical energy -   10 Separation step -   11 Coal -   12 Separation of minerals and amino acids from water -   13 Manure 

1-22. (canceled)
 23. A method for converting biomass into higher-energy-density solids, comprising suspending organic substances from the biomass in water to form a suspension, heating at least a part of the suspension to be converted to a reaction temperature and converting into higher-energy-density solids by hydrothermal carbonization at elevated pressure, wherein said converting is carried out in a reaction volume that is located underneath the Earth's surface.
 24. The method according to claim 23, further comprising buffering released reaction heat of the reaction volume in a surrounding area corresponding to at least four times a mean diameter of the reaction volume by surrounding the reaction volume with a mass of compact liquid and/or solid material which is greater than eight times the biomass contained in the reaction volume.
 25. The method according to claim 23, wherein the converting is carried out at a process pressure which is higher than an equilibrium pressure which would be established at the reaction temperature in a gastight reactor filled with the suspension.
 26. The method according to claim 24, wherein the converting is carried out at a process pressure which is higher than an equilibrium pressure which would be established at the reaction temperature in a gastight reactor filled with the suspension.
 27. The method according to claim 25, further comprising generating the process pressure hydrostatically by introducing the at least a part of the suspension to be converted into a volume region of a volume filled with water or the suspension up to an upper filling level, wherein a height difference of at least 100 m exists between an upper filling level and the volume region forming the reaction volume.
 28. The method according to claim 23, wherein a cavity in the Earth's surface serves as the reaction volume and is filled with water or the suspension.
 29. The method according to claim 23, wherein a region below a water surface of a sea or a lake serves as the reaction volume.
 30. The method according to claim 28, further comprising inserting at least one reactor in the cavity and filling said at least one reactor with water or the suspension.
 31. The method according to claim 29, further comprising inserting at least one reactor in the sea or the lake and filling said at least one reactor with water or the suspension.
 32. The method according to claim 30, wherein the at least one reactor is formed with a flexible outer wall.
 33. The method according to claim 30, wherein the at least one reactor is inserted in a horizontal or inclined shaft and is surrounded by water, which exhibits a hydrostatic pressure whereby a wall of the reactor is at least partially relieved of pressure in the reactor.
 34. The method according to claim 30, wherein the at least one reactor is at least partially perforated to allow passage of gases.
 35. The method according to claim 28, further comprising supplying cold water, for slowing conversion of the biomass, in a controlled manner to the reactor or cavity at different heights via cooling water supply lines to avoid overheating.
 36. The method according to claim 23, further comprising pumping said at least a part of the suspension to be converted in a pumping direction through the reaction volume, to produce a pulsating flow of the at least a part of the suspension to be converted through the reaction volume by repeated brief reversal of the pumping direction.
 37. The method according to claim 23, further comprising generating turbulence in the reaction volume to counter any sedimentation of solids in the reaction volume.
 38. The method according to claim 23, further comprising pumping the suspension water or another cooling medium for cooling and using the heat dissipated by the cooling to generate electrical energy.
 39. The method according to claim 23, further comprising providing the Earth's heat in the reaction volume sufficient to contribute towards increasing the temperature of the suspension to be converted.
 40. The method according to claim 23, further comprising forming a circuit in which the higher-energy-density solids are removed from the suspension after conversion and supplying an added part of the suspension anew to the reaction volume.
 41. The method according to claim 23, further comprising separating heavy substances from the suspension before introducing the suspension into the reaction volume.
 42. The method according to claim 23, wherein the water contains at least one conversion-promoting substance or at least one conversion-promoting substance is added to the water or the suspension.
 43. The method according to claim 23, further comprising adjusting viscosity of the suspension supplied to the reaction volume so that the viscosity is at least 20 mPas.
 44. The method according to claim 23, further comprising adjusting viscosity of the suspension supplied to the reaction volume so that a liquid phase extracted from the reaction volume does not exceed a viscosity of 5 mPas.
 45. The method according to claim 23, further comprising providing the suspension to be converted in a container which allows pressure exerted on the container to be transferred to contents of the container, wherein the suspension to be converted remains in the container during said converting.
 46. Higher-energy-density solids produced by the method according to claim 23 comprising fuel or a starting material for a fuel.
 47. Higher-energy-density solids produced by the method according to claim 23 comprising hydrocarbon-rich liquid fuel. 